Influence of additives on recycled polymer blends
M. A. AlMaaded
•
N. K. Madi
•
A. Hodzic
•
C. Soutis
Received: 9 July 2012 / Accepted: 26 April 2013 / Published online: 11 June 2013
Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract
Polymer systems based on polymer waste offer
promising way to increase recycling in the society. Since
fillers play a major role in determining the properties and
behavior of polymer composites, recycled polymers can
also be combined with fillers to enhance the stiffness and
thermal stability. In this study, blends of recycled poly-
ethylene and recycled polypropylene with mica and glass
fiber were prepared by melt blending technique. The effect
of the particle loading, filler type, and filler–matrix inter-
action on thermal degradation and thermal transition of
processed systems were investigated. Thermogravimetric
analysis, differential thermogravimetric analysis, and dif-
ferential scanning calorimetry were used in this investiga-
tion. Comparative analysis shows that both fillers produced
different effects on thermal properties of the processed
systems. These results were confirmed by calculating the
activation energy for thermal degradation and thermal
transition using Kissinger and Flynn–Wall expressions.
Keywords
Polyethylene blends
Polypropylene blends
Glass fiber-reinforced composites
Mica-reinforced
composites
DSC TG
Introduction
Plastics have become one of the materials with the greatest
growth in terms of consumption and waste generation. The
growth of plastic wastes in UK and Qatar is reaching
20 % of total municipality waste with an average of
2,300 t day
-1
[
]. Therefore, the environmental impact has
become an important challenge for many parts in the
world. Recycling of polymer waste has recently attracted a
considerable attention predominantly due to the increase in
the environmental concerns and the depletion of petroleum
resources [
]. Although the recycling capacity for plas-
tics has been progressively increased, the fraction of
plastics that end up in a landfill is still very significant [
As a consequence, there is a great interest in finding new
possibilities for the use of post-consumer plastics in new
products [
]. Several technologies have been proposed to
solve this problem and to extend the application of poly-
mers into more sectors especially as adopting them as
matrices in new composites.
Polymers have a broad amount of applications such as
packaging products, containers, and consumer goods.
Recycled polymers can be used in a growing number of
potential applications, such as boxes or pallets; however,
deterioration in thermal and mechanical properties of
polymers during repeated thermal processing limit the
applications of recycled polymers and their blends [
].
During successive thermal processing steps, irreversible
thermo-oxidative degradation in the form of chain scission,
cross-linking or elimination of substituent, and formation
of double bond can be produced, yielding recycled mate-
rials with lower properties than that of the original batch
[
]. To avoid degradation, additives like compatibilities
and antioxidants are added to contribute to the preservation
of the original properties [
]. However, their introduction
M. A. AlMaaded (
&) N. K. Madi
Center of Advanced Materials (CAM), Qatar University,
Doha, Qatar
e-mail: m.alai@qu.edu.qa
A. Hodzic
C. Soutis
Composite Systems Innovation Centre,
Department of Mechanical Engineering,
The University of Sheffield, Sheffield, UK
123
J Therm Anal Calorim (2014) 115:811–821
DOI 10.1007/s10973-013-3224-y
normally leads to an increase in the cost of the materials.
To improve the processing, polymer composites retaining
the good thermal and mechanical properties have been
used. Incorporating inorganic fillers [
] into polymer resin
improves various materials properties such as strength and
thermal stability.
Polyolefin filled with inorganic materials has gained the
confidence of end users because it provides excellence
performance, recyclability, design, and fabrication flexi-
bility. Adding the fillers has the effect of reducing the cost
and enhancing mechanical and thermal properties of
polymer composites and, therefore, is favorable for poly-
mer engineering. As a result significant number of filled
and reinforced polyolefin grades have been developed and
successfully used in various applications in automotive,
building and construction, electronics, sport equipment,
furniture, and appliance market. The most common fillers
of PE are calcium carbonate [
], others like mica, glass
beads [
], and clays [
] have been successfully
used. Among traditional fillers, mica is of special interest
because it can promote heterogeneous crystallization in
semicrystalline polymers leading to a significant change in
properties. Generally, these properties of inorganic filler–
polymer composites depend strongly on size, shape, and
distribution of filler in the polymer matrix and also to the
extent of interfacial adhesion between the filler and the
matrix.
Most polymer composites are generally subjected to
thermal degradation at elevated temperatures. The practical
use of composites requires knowledge of their thermal
lifetime, which corresponds to their upper limit and the
operating temperatures as well as thermal stability of these
materials. Materials having high thermal stability can
retain their mechanical integrity when exposed to elevated
temperatures [
]. It is important, therefore, to understand
the effect of the processing temperature on the thermal
degradation process, since the polymers are exposed to
thermal degradation during the manufacturing of filler-
reinforced composites. Thermal analysis is an important
technique for determination of the thermal stability of the
materials [
]. Moreover, it is a useful technique for
understanding structure–property relationship. In addition,
it is possible to quantify the amount of moisture and vol-
atiles, which can cause deterioration of the composite
properties [
]. Different fillers-modified polymer
samples were prepared in the present study by varying the
proportion of the filler content as well as the filler type.
Thermogravimetric analysis (TG), differential scanning
calorimetry (DSC), and morphological systems were used
to investigate the effect of filler type, filler loading, and
filler–matrix adhesion on the thermal degradation, thermal
transition, and morphology of the thermoplastic compos-
ites. The DSC and TG results obtained for the studied
composites were analyzed using Kissinger [
] and Flynn–
Wall–Ozawa [
,
] kinetic methods, respectively, as
detailed in the following sections.
Theoretical approach
A polymer has a stable molecular structure that vaporizes,
sublimates, or its molecular structure becomes brittle at a
high temperature to decompose into gaseous product [
This process of molecular degradation may change the
mass of the sample as a function of temperature. Since the
thermal decomposition has a direct correlation with the
stability of the composites, TG was used to measure the
activation energy E
a
of composites to determine its
stability.
The fundamental rate equation used in all kinetic studies
is generally described as
da
dt
¼ kf a
ð Þ
ð1Þ
where k is the rate constant and f
ðaÞ is the reaction model,
a function depending on the actual reaction mechanism.
Equation (
) expresses the rate of conversion, da/dt, at
a constant temperature as a function of the reactant
concentration loss and rate constant. In this study, the
degree of conversion or degradation of the material a can
be calculated by the TG curve as:
a
¼
W
o
W
t
ð
Þ
W
o
W
f
ð
Þ
ð2Þ
where W
t
, W
o
, and W
f
are time related (t), initial, and final
mass of the sample, respectively. The rate constant k is
generally given by the Arrhenius equation:
k
¼ A exp E=RT
ð3Þ
where E is the apparent activation energy (kJ mol
-1
), R is
the gas constant (8.314 J K
-1
mol
-1
), A is the pre-
exponential factor (min
-1
), and T is the absolute
Temperature (K). Combining Eqs. (
) and (
) gives the
following correlation:
da
dt
¼ A exp E=RT
f
a
ð Þ
ð4Þ
For a dynamic TG technique, introduce the heating rate,
b = dT/dt, into Eq. (
), and get Eq. (
):
da
dT
¼ A=b
exp E=RT
f
a
ð Þ
ð5Þ
Equations (
) and (
) are the fundamental expression of
analytical methods to calculate kinetic parameters on the
basis of TG data. The most common ‘‘model free’’ methods
used in our study are tabulated in Table
812
M. A. AlMaaded et al.
123
Experimental
The materials used in this study were recycled polyethyl-
ene and recycled polypropylene supplied in pellets by local
plastic recycling plant (
, Qatar),
which were derived mainly from post-consumer plastics
waste. Both materials were characterized and some phys-
icochemical characteristics are summarized in Table
Mica flakes and chopped glass fibers provided by Sheffield
University were used as fillers with their respective prop-
erties presented in Table
. The materials were thoroughly
dried before the compounding in order to minimize the
influence of moisture. Polypropylene and polyethylene
were dried at 60
°C for one hour; the glass fiber and mica
were dried at 90
°C for three hours. Systems made of
recycled polyolefin and combinations of fillers were pre-
pared using Brabender a twin-screw extruder at Qatar
University. The processing temperature of extruder in the
feeding zone, the mixing zone, and the die were 180, 200,
and 220
°C, respectively. The twin screw rotating speed
was fixed at 60 rpm. Upon completion of melt blending,
the extruded strands were allowed to cool in the water bath
at 25
°C, and then cut into pellets using a pelletizer.
Table
shows the designated symbol for each system and
the corresponding compositional ratio of each constituent.
Thermogravimetry
Thermogravimetric analysis can be used to evaluate the
thermal behavior and stability parameters of the neat and
modified thermoplastics [
]. Thermogravimetric (TG) and
differential thermogravimetric (DTG) measurements were
conducted with Perkin Elmer Pyris Analyzer 6 at different
heating rates. Heating rates of 5, 10, 20, and 40
°C min
-1
were used. Samples (*10 mg) were placed in a ceramic
pan and the experiments were conducted in nitrogen
environment with N
2
being supplied at a flow rate of
20 mL min
-1
. In this work, detailed and precise factors
defining the thermal stability based on the onset tempera-
ture of thermal decomposition (T
onset
) (at conversion fac-
tor, a = 5 %), temperature of maximum rate of mass loss
(T
peak
) (from DTG curves), the end temperature at which
the residue remains unaffected (T
end
), the residue which
presented the residual solid mass fraction percentage
detected at T
end
, and the activation energy (E
a
) were
investigated. The activation energies were calculated by
Flynn–Wall–Ozawa (Integral method) and Kissinger.
Conversion values, a = 5–50 % at each 5 % conversion
were used for estimating the activation energy values using
Flynn–Wall–Ozawa method.
Differential scanning calorimetry
Differential scanning calorimetry analysis was carried out
using a Perkin Elmer Instrument Pyris 6 DSC with a
sample mass of 8–10 mg. All samples were held at 30
°C
for 5 min, heated at a rate of 10
°C min
-1
to 300
°C,
subsequently held for 5 min to erase the previous thermal
history, and then cooled at a rate of 10
°C min
-1
to 30
°C,
subsequently held for 5 min and heated again at a rate of
10
°C min
-1
to 300
°C under nitrogen atmosphere. The
cold crystallization temperature (T
c
), melting temperature
(T
m
), and heat of fusion were determined from the second
heating scan. The crystallinity of samples (X
c
) was deter-
mined using the following expression [
X
c
¼
DH
f
100
DH
f
w
ð6Þ
The value of DH
f
which is the heat of fusion of 100 %
of crystalline RPE is 293 J g
-1
[
] and 209 J g
-1
for RPP
], DH
f
is the heat of crystallization obtained from DSC
Table 1
Kinetic methods used in our study to evaluate activation energy
Method
Expression
Plots
References
Kissinger
ln
b
T
2
p
¼ ln Af a
ð Þ
½
E
a
RT
p
ln
b
T
2
p
vs: 1=T
p
Flynn–Wall–Ozawa
logb
¼ log
AE
a
Rg
a
ð Þ
2:315
0:457E
a
RT
log b vs:1=T
Table 2
Some physicochemical characteristics of the recycled
polyethylene, recycled polypropylene, mica and glass fibers used
Recycled polyethylene
Density/g cm
-3
0.92
Melt flow index (g/10 min, 190
°C, 2.16 kg)
0.5
Ash/mass%
1–2
Recycled polypropylene
Density/g cm
-3
0.94
Melt flow index (g/10 min, 190
°C, 2.16 kg)
1.55
Ash/mass%
1–2
Mica flakes
Aspect ratio
1–10
Density/g cm
-3
2.9
Chopped glass fibers
Average length/mm
4.5
Average size/lm
14
Density/g cm
-3
2.9
Additives on recycled polymer blends
813
123
curves, and w is the mass fraction of polymer in the
composite.
Results and discussions
Thermogravimetry
Effect of particle loading on thermal degradation
The thermal degradation of neat recycled polyethylene
(RPE), and the systems of RPE containing different
quantities of mica in nitrogen was determined by thermo-
gravimetry. The mass loss increases with temperature, for
heating rate of 10
°C min
-1
, are shown in Fig.
a, b. The
onset temperature of thermal degradation (T
onset
), peak
mass loss temperature (T
peak
), end decomposition temper-
ature (T
end
), and the residue at T
end
are given in Table
These results are obtained by averaging the results from
three runs for each sample. T
onset,
T
peak
, T
end
, and the res-
idue in Table
are the average of three values. A deriva-
tive mass loss curves, DTG, were used to indicate the
temperatures at which mass loss is peak (T
peak
) and ends
(T
end
).
It is evident from TG curves that, the mass loss of RPE
occurred in a one-step degradation process from 400
°C to
below 500
°C. This result is confirmed by the presence of
single peak in DTG curve, at temperature of 478.3
°C
(Table
). The mass loss of RPE starts at about 400
°C.
Above 450
°C, this process accelerates rapidly and the
quantity of RPE residue is relatively low (equal to 4.8 %).
This is due to further break down of RPE into gaseous
products at high temperature. The relatively higher amount
of residue in RPE is due to thermally stable additives that
were added during the recycling process.
On the other hand, the thermal degradation of RPE
composites with mica increases gradually up to approxi-
mately 400
°C, and then a more marked mass loss occurs
between 400 and 500
°C. All samples showed a peak mass
loss on reaching about 480
°C more or less (Table
). This
result is also confirmed by the presence of the single peak
from DTG curves, which might have been the result of
random chain scission process and subsequent pyrolysis of
the samples.
The effect of particle loading can be observed also from
Fig.
a, b, in which the thermal stability of recycled RPE is
sensitive to a large quantity of inorganic fillers. This
clearly appears in the obtained values of T
onst
, T
peak
, and
T
end
(Table
). The onset temperatures of the thermal
Table 3
Compositions of the processed samples
Designated symbol
Composition and compositional ratio
By volume/vol %
By mass/mass %
MRPE1
15 mica/85 RPE
35 mica/65 RPE
MRPE2
30 mica/70 RPE
57 mica/43 RPE
MRPP2
30 mica/70 RPP
43 mica/57 RPE
GFRPE1
15 GF/85 RPE
32 GF/68 RPE
GFRPP1
15 GF/85 RPP
33 GF/67 RPP
M mica, GF glass fiber, R recycled, PE polyethylene, PP
polypropylene
100
MRPE1
MRPE2
RPE
MRPE1
MRPE2
RPE
80
60
40
20
0
400
400
420
440
460
480
500
520
540
450
Temperature/°C
Temperature/°C
Derivative mass loss/% min
–1
Mass loss/%
500
600
550
5
0
–5
–10
–15
–20
–25
–30
(a)
(b)
Fig. 1 a
TG curves of RPE and MRPEs systems at heating rate
10
°C min
-1
and b the corresponding DTG curves
Table 4
Decomposition characteristics of pure RPE, pure RPP,
MRPE1, MRPE2, MRPP2, GFRPE1, and GFRPP1 systems
Composite
T
ons
/
°C
T
peak
/
°C
T
end
/
°C
Residue/%
RPE
429.0
478.3
497.1
4.8
RPP
415.5
459.8
493.0
7.0
MRPE1
428.3
479.8
496.8
29.0
MRPE2
442.8
483.1
500.8
51.9
MRPP2
442.6
473.4
479.8
56.0
GFRPE1
434.9
479.4
495.5
32.3
GFRPP1
404.9
458.2
478.7
40.1
814
M. A. AlMaaded et al.
123
degradation processes of the system that contains higher
content of fillers (30 % of mica) were increased compared
to that of system containing 15 % of mica. For T
peak
and
T
end
of thermal degradation process of studied systems, the
temperature difference between low and higher content is
slightly changed. Decomposition temperature increases
with the increase in filler content in the system due to the
relatively high molecular weight and crystalline nature of
the modifier, which increases their thermal resistance.
Effect of filler type on the thermal degradation of systems
These experiments were designed to measure the effect of
GF loading on the thermal properties of RPE in comparison
to the effect of mica. This was examined in non-isothermal
TG and DTG. The TG and the corresponding DTG curves
obtained at a heating rate of 10
°C min
-1
for GFRPE1 and
MRPE1 are shown in Fig.
a, b. TG curves correspond to
single-stage degradation with well-defined initial and final
degradation temperatures. The rapid mass loss steps of both
systems start at 430
°C. The thermal degradation of samples
can take place through random chain scission and a radical
chain mechanism. The onset temperature of thermal deg-
radation (T
onset
), the peak temperature where the maximum
decomposition rate was obtained (T
peak
), the end tempera-
ture at which the residue remains unaffected (T
end
), and the
residue which presented the residual solid mass fraction
percentage detected at T
end
are tabulated in Table
. Evi-
dently, the onset temperature of thermal degradation of the
GFRPE1 system is higher than that of MRPE1. This is
indicative of improved fiber–matrix interaction and the
effectiveness of glass fiber as reinforcing agent.
It is also seen from Table
that even after complete
degradation, char residue is very high for both systems
filled with GF and mica whereas for neat RPE char residue
is only 5 %. This is due to the presence of thermally stable
glass fiber and solid inorganic filler. Moreover, the residue
solid of both systems were found to be consistent with the
composition in the molding compounders suggesting good
dispersion of the fillers in the matrix.
Effect of polymer–filler interaction on the thermal
degradation of systems
Figure
a shows the thermal stability when mica is added
to RPE and RPP. While, Fig.
b shows when GF is added
to RPE in comparison to GF-filled RPP. In GF- and mica-
filled RPP, the decomposition temperature and the thermal
stability are reduced as shown in Fig.
a, b and presented
in Table
. Also, the shifting of the peak values of (T
onst
),
(T
peak
), and (T
end
) is observed with GF-filled RPE in
comparison to GF-filled RPP. Same trend is observed for
(T
peak
) and (T
end
), while there is no significant change in
(T
onset
), when comparing thermal degradation of RPE
composite with mica and mica-filled RPP system. Appar-
ently, there is little compatibility and interfacial adhesion
between inorganic fillers and RPP matrix. Some studies
have reported [
] that the uncoupled glass fiber creates a
poor interfacial adhesion with polypropylene resin.
Determination of activation energy
F–W–O method
TG curves for the MRPE1, MRPE2, MRPP2, GFRPE1,
and GFRPP1 systems at different four heating rates are
shown in Fig.
a–e, respectively. It is observed that the
increase in the heating rate results in a higher degradation
temperature, and a higher decomposition temperature
(T
onst
). The possible explanation is that the polymer
absorbs more heat energy during exposure to slow heating
rates [
]. To obtain activation energy (E
a
kJ mol
-1
) of
thermal decomposition using Flynn and Wall expression
100
80
60
40
20
0
Mass loss/%
400
450
Temperature/°C
500
600
550
400
450
Temperature/°C
500
600
550
Derivative mass loss/% min
–1
5
0
–5
–10
–15
–20
MRPE1
GFRPE1
(a)
(b)
Fig. 2 a
TG curves of MRPE1 and GFRPE1 systems at 10
°C min
-1
and b DTG curves for the same systems
Additives on recycled polymer blends
815
123
[
], the first step is the choice of level of decompo-
sition. Typically, a value early in the decomposition profile
is desired since the mechanism is more likely to be that of
the actual product failure. On the other hand, taking the
value too early on the curve may result in the measurement
of some volatilization (e.g., moisture) which is not
involved in the failure mechanism. A value of 5 % con-
version level is a commonly chosen value. Therefore, the
fixed conversions, a were selected from 0.05 with incre-
ment of 0.05 until the temperature at which the mass loss
change remains stable. After the final conversion level,
there is only the presence of residue not considered in the
calculation of E
a
. Using the selected value of conversion,
the temperature (in Kelvin) at that conversion level is
measured for each thermal curve. A plot of log b versus
1,000/T at constant conversion is produced. The plotted
data should produce a straight line, and the activation
energy is determined from the slope of each line. Inset of
Fig.
shows a series of such lines created from the six
curves shown in Fig.
a–e, respectively, by plotting the
data at different conversion levels.
It is shown in the insets of Fig.
a–e that the fitted lines
are nearly parallel, which indicates approximate activation
energy values at different conversion rates, and conse-
quently implies the possibility of single reaction mecha-
nism (or the unification of multiple reaction mechanisms).
The reaction mechanism, however, also might change in
comparatively higher conversion periods according to
slopes at a = 0.6 in Fig.
a–e. The change of reaction
mechanism in higher conversion might be caused by the
complex reactions in decomposition process of the main
filler components. Generally, the decomposition conver-
sion when conversion rate is higher than 0.6 becomes
meaningless for polymer systems due to high temperature
and sample mass loss.
Figure
shows the plots of activation energy E
a
as a
function of conversion a obtained for the thermal degra-
dation of RPE, MRPE1, MRPE2, MRPP2, GFRPE1, and
GFRPP1
systems
calculated
by
Flynn–Wall–Ozawa
method. As the thermal decomposition process proceeded
in the inorganic filler, the activation energy was slightly
changed after the initial stage and then remained nearly
constant within a certain range of a. This range is system
dependent and after that range, E
a
was slightly decreased.
Also shown, the presence of low values of E
a
for low
conversion and high E
a
in high conversion might imply the
different decomposition mechanisms in the whole process.
Whenever filler is added into a polymer, the interaction
between polymer and filler may lead to many processes
like bound rubber formation, rubber shell formation,
occlusion, and filler networking. Based on the nature of
filler, polymer, and particle loading one or more of the
above process can take place and have effect on the deg-
radation of the polymer [
–
Isoconversional Kissinger method
Free isoconversional Kissinger method [
] is used to cal-
culate the activation energy as an alternative way in this
study. The maximum degradation temperature (T
peak
) for
different systems has been determined first from DTG curves
of each system and hence ln(b/T
peak
) against 1,000/T
peak
plots are prepared and presented in Fig.
. Figure
shows
that the fitting straight lines are nearly parallel and thus
improve the applicability of this method. The activation
energy is determined from the slope of the straight line.
DSC analysis
DSC has been carried out to evaluate interaction of par-
ticulate components in the polymer composites. The effect
100
80
60
40
20
0
Mass loss/%
100
80
60
40
20
0
Mass loss/%
400
450
Temperature/°C
500
600
550
400
450
Temperature/°C
500
600
550
MRPE2
MRPP2
GFRPE1
GFRPP1
(a)
(b)
Fig. 3 a
TG curve of MRPE2 in comparison to that of MRPP2 and
b
TG curve of GFRPE1 in comparison to TG curve of GFRPP1 (at
10
°C min
-1
)
816
M. A. AlMaaded et al.
123
of filler type and filler content on the T
m
, T
c
, DH
f
, and
crystallinity (X
C
) of each system obtained from Fig.
a, b
are illustrated in Table
. The endotherm between 100 and
140
°C and between 150 and 180 °C represents the melting
of the crystallites in the RPE and RPP matrix, respectively.
As shown in Fig.
a, b, DSC curves of RPE revealed
one sharp melting peak at 127.8
°C and peak shoulder at
110
°C. These indicated the presence of different types of
polymer grades in plastic waste. Evidently from Fig.
a, b,
the heat of enthalpy of melting peak at 127.8
°C is pre-
valent as compared to the heat of enthalpy of the shoulder
peak at 110
°C, which designates that the waste plastic
contains major percentage of high molecular mass grades.
Also, the incorporation of mica-filled RPE has significant
effect on both T
m
and T
c
(Table
). Two melting endo-
therms still have been observed for mica-filled RPE sys-
tem, one sharp around 130
°C and a broad shoulder peak
appears at about 110
°C. This shoulder peak becomes
100
80
60
40
20
0
Mass loss/%
100
80
60
40
20
0
Mass loss/%
log
β
log
β
log
β
log
β
log
β
100
120
80
60
40
20
0
Mass loss/%
100
80
60
40
20
0
Mass loss/%
100
80
60
40
20
0
Mass loss/%
400
450
Temperature/°C
1000/T/K
–1
500
550
400
450
Temperature/°C
1000/T/K
–1
500
550
400
450
Temperature/°C
1000/T/K
–1
500
550
1.3
1.4
1.5
1.6
1.2
1.3
1.4
1.5
400
450
Temperature/°C
1000/T/K
–1
500
550
1.2
1.3
1.4
1.5
1.25
1.30
1.35
1.40
1.45
1.50
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
400
450
Temperature/°C
1000/T/K
–1
500
550
1.30
1.35
1.40
1.45
1.50
β = 5 °C min
α = 5 %
α = 10 %
α = 15 %
α = 20 %
α = 25 %
α = 30 %
α = 35 %
α = 40 %
α = 5 %
α = 10 %
α = 15 %
α = 20 %
α = 25 %
α = 30 %
α = 35 %
α = 40 %
α = 45 %
α = 50 %
α = 55 %
α = 60 %
α = 5 %
α = 10 %
α = 15 %
α = 20 %
α = 25 %
α = 30 %
α = 35 %
α = 40 %
α = 45 %
α = 50 %
α = 55 %
α = 5 %
α = 10 %
α = 15 %
α = 20 %
α = 25 %
α = 30 %
α = 35 %
α = 40 %
α = 45 %
α = 50 %
α = 55 %
α = 5 %
α = 10 %
α = 15 %
α = 20 %
α = 25 %
α = 30 %
α = 35 %
–1
β = 10 °C min
–1
β = 20 °C min
–1
β = 40 °C min
–1
β = 5 °C min
–1
β = 10 °C min
–1
β = 20 °C min
–1
β = 40 °C min
–1
β = 5 °C min
–1
β = 10 °C min
–1
β = 20 °C min
–1
β = 40 °C min
–1
β = 5 °C min
–1
β = 10 °C min
–1
β = 20 °C min
–1
β = 40 °C min
–1
β = 5 °C min
–1
β = 10 °C min
–1
β = 20 °C min
–1
β = 40 °C min
–1
(a)
(b)
(c)
(d)
(e)
Fig. 4
TG curves for a MRPE1, b RPE2, c MRPP2, d GFRPE1, and e GFRPP1 systems at different heating rates. The insets are the
isoconversional curves of each system by F–W–O expression derived from the corresponding figures
Additives on recycled polymer blends
817
123
less pronounced when mica content increases by 30 % by
volume indicating that the bimodal crystal distribution
disappears by increasing the filler content. Furthermore, the
width of the cold crystallization and melting curves for
both systems is narrow and becomes narrower at higher
filler content. Thus, in the case of mica-filled RPE, it seems
that there is some level of compatibility in these systems
especially at higher content of the filler. The increase in
perfection or crystallinity (Table
) is due to enhancement
of crystal nucleation in the region surrounding the rein-
forced particles [
]. This is strongly recommended by the
observed decrease in T
c
values (Table
) for these two
systems indicating the mica as an effective nucleating
agent.
The results were different for RPP and its composite
with mica. As shown in Fig.
b, two melting peaks have
been observed, one sharp around 160
°C and a second
smaller melting peak appears at about 150
°C. This would
indicate that mica-filled RPP system exhibited a bimodal
crystal distribution after processing. This also appears in
the low crystallinity level obtained after incorporation of
mica into RPP when compared to pure RPP crystallinity
(Table
In addition, Fig.
a, b shows DSC thermograph of RPE
composite with GF. Table
shows the variation of melting
temperature, crystallization temperature, and crystallinity
of polymer matrix with GF addition. Compared with the
pure polymer, the new morphology has not changed the
position of melting peak and cold crystallization signifi-
cantly. The peaks shape has changed only due to the
presence of GF filler. As shown in Table
the crystallinity
of GF-reinforced RPE is increased predominantly. This
increment is due to the small and uniform crystallite size
220
215
210
205
200
195
190
185
RPE
MRPE1
MRPE2
MRPP2
GFRPE1
GFRPP1
180
Conversion rate/%
E
a
/kJ mol
–1
0
20
40
60
0
20
40
60
0
20
40
60
Fig. 5
A comparison of apparent activation energy as a function of
decomposition conversion rate (a) for all investigated composites
calculated by F–W–O method. The solid lines are drawn to guide the
eyes
–9.0
MRPE1
MRPE2
MRPP2
GFRPE1
GFRPP1
–9.5
–10.0
–10.5
–11.0
–11.5
–12.0
1.28 1.30 1.32 1.34 1.36 1.38
1.28
1000/T
m
/K
–1
ln(
β
/T
m
2
)
1.30 1.32 1.34 1.36 1.38 1.40 1.42
Fig. 6
Kissinger non-isothermal plot of heating rate versus reciprocal
of temperature for MRPE1, MRPE2, MRPP2, GFRPE1, and GFRPP1
at DTG peak temperature
40 60
RPE
RPP
MRPE1
MRPE2
MRPP2
GFRPE1
GFRPP1
80 100 120 140 160 180 200
100 120 140 160 180 200
40
Temperature/°C
Heat flow endo up/mW
60 80
(a)
(b)
Fig. 7
DSC endothermic melting curves and exothermic cold
crystallization for a RPE, MRPEs, GFRPE1, and b RPP, MRPP2,
and GFRPP2 systems at heating rate of 10
°C min
-1
Table 5
DSC curves data of RPE, RPP, MRPEs, MRPP2, GFRPE1,
and GFRPP1 systems at heating rates of 10
°C min
-1
Composite
T
m
/
°C
T
c
/
°C
(DH
f
)
polymer
/J g
-1
(X
f
)
polymer
/%
RPE
127.4
114.8
98.7
33.7
MRPE1
131.1
111.8
106.5
36.4
MRPE2
130.1
112.5
120.7
41.2
GFRPE1
128.6
114.2
120.0
41.0
RPP
162.0
119.1
80.0
38.3
MRPP2
164.9
122.0
46.3
22.2
GFRPP1
166.6
123.5
54.2
26.9
818
M. A. AlMaaded et al.
123
distribution. Also it indicates that there is a good adhe-
sion between GF and RPE matrix. The size of crystallites
is determined by the type and volume fraction of added
filler [
Compared with the neat RPP, the composite with GF
has higher melting peak temperature and also crystalliza-
tion peak temperature. Since untreated GF does not exhibit
good adhesion or dispersion in the RPP matrix, one
observes the deterioration in the crystallinity of RPP
composite with GF (Table
). However, the final statement
about the effect of filler on thermal stability requires more
study of thermal history and filler characteristics which will
be presented in the future work.
Determination of activation energy for investigated systems
Since
many
transitions
(evaporation,
crystallization,
decomposition, etc.) are kinetic events, they are function of
both time and temperature. DSC, therefore, was used with
four scanning rates (5, 10, 20, and 40
°C min
-1
) to ana-
lyze, MRPE1, MRPE2, MRPP2, GFRPE1, and GFRPP1
systems and compare their characteristics for thermal
transition. The effects of heating rate on melting behavior
of studied systems are shown in Fig.
a, b. It can be seen
that the lower the heating rate, the higher the peak
45
(a)
(b)
MRPE1
MRPE2
MRPP2
GFRPE1
GFRPP1
40
35
30
25
20
15
40
80
120
160 40
80
120 160
40
80
120 160
80
120
160
200
80
120
160
200
Temperature/°C
Temperature/°C
Temperature/°C
Heat flow undo up/mW
Heat flow undo up/mW
Heat flow undo up/mW
β
= 5 °C min
–1
β
= 10 °C min
–1
β
= 20 °C min
–1
β
= 40 °C min
–1
β
= 5 °C min
–1
β
= 10 °C min
–1
β
= 20 °C min
–1
β
= 40 °C min
–1
Fig. 8 a
DSC curves for MRPE1, MRPE2 and MRP2 systems and b GFRPE1, and GFRPP1 systems at different heating rates (5, 10, 20, and
40
°C min
-1
)
–9.0
R
2
= 0.909
R
2
= 0.990
R
2
= 0.824
R
2
= 0.938
R
2
= 0.995
–8.5
MRPP2
MRPE2
MRPE1
GFRPE1
GFRPP1
–8.0
–9.5
–10.0
–10.5
2.44
2.46
2.48
2.50
2.40
2.44
2.48
2.40
2.44
2.48
2.24
2.26
2.28
2.30
2.52
ln(
β
/T
m
2
)
ln(
β
/T
m
2
)
–9.0
–8.5
–8.0
–9.5
–10.0
–10.5
ln(
β
/T
m
2
)
–9.0
–8.5
–8.0
–9.5
–10.0
–10.5
ln(
β
/T
m
2
)
1000/T
m
/K
–1
1000/T
m
/K
–1
1000/T
m
/K
–1
1000/T
m
/K
–1
(a)
(b)
Fig. 9
Kissinger non-isothermal plot of heating rate versus reciprocal of temperature for a MRPE1, MRPE2, and MRPP2 systems and
b
GFRPE1, and GFRPP1 at DSC peak (B) temperature derived from Fig.
a, b, respectively
Table 6
The value of E
a
/kJ mol
-1
for the polymer degradation in
each system obtained by Kissinger method from TG analysis and
compared with average values of activation energy obtained by F–W–
O methods obtained at different conversion ratios
System
Activation energy/kJ mol
-1
obtained by
F–W–O method
Kissinger method
TG
TG
DSC
RPE
h207.7i ± 9
90.4
–
MRPE1
h213.9i ± 29
268.9
372.6
MRPE2
h236.3i ± 43.4
295.8
458.1
MRPP2
h228.9i ± 40.7
227.5
233.1
GFRPE1
h244.8i ± 68.3
320.5
221.0
GFRPP1
h214.0i ± 19.9
247.9
87.3
The values of E
a
obtained by Kissinger method from DSC analysis
are also presented
Additives on recycled polymer blends
819
123
resolution, and consequently the shoulder peak still exists.
This may be interpreted that the slower heating rates enable
a semicrystalline polymer to have more time for crystal
growth prior to final melting. At higher heating rate, there
is no sufficient time for crystal rearranging and the shoul-
der peak, representing two populations of RPE and RPP
crystals in the polymer composites, disappeared. For this
reason, high heating rates should normally be used when
trying to measure small transitions as they provide large
heat flow signals. It is also shown that distinguishing peaks
are shifted to higher temperatures with an increase in
heating rate [
].
The peak temperatures of the main endothermic curves
in the DSC signals at different heating rates were used with
the Kissinger model for determination of the activation
energy of thermal transition which can be calculated from
the slope of the graph by plotting ln (b=T
2
m
) as a function of
(1,000/T
m
) [
]. Figure
a, b shows the Kissinger plots for
the studied systems.
The values of E
a
for the polymer degradation in each
system obtained by Kissinger method from TG analysis are
presented in Table
and compared with average values of
activation energy obtained by F–W–O methods obtained at
different conversion ratios. It is noticed that the values
obtained by Kissinger’s method are overall higher than
those obtained by isoconversional method. Accordingly, an
appropriate apparent activation energy range should be
obtained by combining all values from the two methods
and consequently a general activation energy range is
suggested for most inorganic fillers for the purpose of
polymer composites processing. The obtained values of
activation energy by Kissinger method from DSC analysis
are also presented in Table
and, therefore, can be com-
pared with the corresponding values obtained from TG
analysis.
Conclusions
In this study, detailed experimental analyses of thermal
properties of recycled PE and PP with two types of inor-
ganic fillers (glass fiber and mica) commonly used in the
polymer composite industry were carried out. Composites
were made by melt compounding. The thermal properties
of these composites were studied using TG and DSC
techniques and the conclusions can be summarized as
follows.
(1)
Use of recycled polymer composites serves the dual
purpose of enhancing materials performance and
providing means for recycling of plastic waste, and
is a more favored disposal solution compared to other
options such as incineration and landfill.
(2)
TG and DTG results revealed that:
(a)
TG curves of RPE and RPP and their composites
with mica and GF showed a single stage of mass
loss. The severe mass loss from 400 to 500
°C is
due to break down of polymer into gaseous
product.
(b)
MRPE2 showed better thermal stability com-
pared with MRPE1 indicating that the effect of
filler reinforcement increased with filler loading.
(c)
The glass fiber is more effective as reinforcing
agent than mica particles.
(d)
Inorganic organic filler exhibited weak interac-
tion with RPP matrix than that observed with
RPE polymer.
(e)
With the TG carried out at four different heating
rates, the F–W–O method was used to analyze
the pyrolysis kinetics of systems. It was found
that the presence of low values of E
a
for low
conversion and high E
a
in high conversion might
imply the different decomposition mechanisms
in the whole decomposition process. It was also
found that higher heating rate provides the better
thermal stability, resulting from the decelerated
decomposition rate.
(3)
DSC analysis revealed that:
(a)
The melting endotherms of RPE and its com-
posite with mica can be considered as bimodal.
Also, mica has the influence on crystallinity
degree of RPE matrix. These trends are depen-
dent on loading level.
(b)
GF promotes the crystallinity degree of RPE
matrix.
Acknowledgments
The authors acknowledge the financial support
from Qatar Science and Technology Park (QSTP). We would like also
like to thank Center of Advanced Materials and office of research at
Qatar University for their support.
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Document Outline
- Influence of additives on recycled polymer blends
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